A.1. Overview. The prevalence of childhood and adolescent obesity is increasing at alarming rates. The prevalence of overweight among adolescents aged 12 to 19 has tripled between 1980 and 2004 (Ogden et al., 2006). The obesity rates are highest among African American and Hispanic youth (National Center for Disease. Statistics, 2004). Co-morbid conditions are increasingly seen in obese youth, including insulin resistance, type 2 diabetes, hypertension, obstructive sleep apnea, poor self-esteem, lower health-related quality of life (Rashid &Roberts, 2000;Rosenbloom, 2002;Sorof, Lai, Turner, Poffenbarger, &Portman, 2004;Wing et al., 2003). Moreover, these conditions tend to track into adulthood for 80% of obese youth (Whitaker, Wright, Pepe, Siedel, &Dietz, 1997). Given the rapidly growing prevalence of obesity in youths, developing evidence-based weight loss treatments that not only achieve weight loss but facilitate sustainability of healthy weight are needed, particularly for low income racially and ethnically diverse populations where the prevalence of overweight is highest. Novel approaches to promoting sustained healthy weight control through behavioral strategies that lead to improvements in adequate physical activity participation in youths are a critical area of research that has not been adequately tested through well-controlled trials. We will test the use of an integrated in-person and Internet-based program that will target increased physical activity and decreased sedentary behavior in overweight boys and girls from the Boston area. The program will utilize behavioral strategies including self-monitoring, goal setting, social support, problem solving and others to promote sustained health behavior change in adolescents. Technology may be a particularly effective tool for delivering such interventions because interactive technologies: 1) are routinely used throughout daily life by most adolescents, for example, the 2004 Boston Youth Survey indicated that 76% of boys and 72% of girls from a ethnically and racially diverse sample have access to the Internet at home (BYS, 2004), 2) have been shown in preliminary studies to increase health behavior change in youth and adults (c.f.., Williamson et al., 2005), and 3) can be delivered in a cost efficient way to large numbers of participants. Several reviews of the literature on technology-based intervention to address adolescent weight control have identified a number of limitations including 1) small sample sizes, 2) inadequate measures of body composition, 3) inattention to the impact of physical development in the analyses, 4) failure to disaggregate results for boys and girls separately , and 5) lack of follow-up (Atlantis, Barnes, Fiatarone, &Singh, 2006;Boon &Clydesdale, 2005;Snethen, Broome, &Cashin, 2006). Of the studies that specifically tested Internet-based programs, all but two studies were quasi-experimental in design, both had random assignment to groups. Moreover, the use of theory was limited in these studies, an important component. Finally, the outcome measures in previous studies were self-report and the sample sizes were typically small. Thus, while preliminary results are promising for the use of technology for physical activity and nutrition promotion programs in youth, there is a critical need to rigorously test the efficacy of an Internet-based program to aid in the adoption and maintenance of weight loss in a trial that is well-designed, with objective outcome measures, adequately powered, theoretically-based, and that has implications for weight loss on a larger-scale. We propose to design an innovative program for weight loss by targeting increased physical activity and decreased sedentary time. Specifically, we intend to test a 3-month supervised exercise program that also includes comprehensive, theoretically-based, in-person and Internet behavioral coaching to promote physical activity for weight loss (Exercise + Internet Coaching) versus a 3-month standard supervised exercise program (Exercise). In this way, we will be able to test whether or not the addition of the in-person and Internet behavioral coaching assists in the achievement and maintenance of a reduction in body fat via promotion of physical activity and a reduction in sedentary time.